The tech race to eliminate aircraft emissions

Several experiments are underway, but to date nothing approaching the scale needed to replace jet fuel entirely

By Gunnar Wetlesen

(Gunnar Wetlesen is a semiconductor veteran who began making chips when integrated circuits had only hundreds of transistors, and he went on to co-found two successful IC companies. The second, VLSI Technology Inc., pioneered the present design and foundry model. Gunnar is passionate about science and technology, especially innovations that promote sustainability and address climate change. He is a consultant and adviser and has a degree in physics.)

SAN FRANCISCO (Callaway Climate Insights) — What are the paths to no or neutral greenhouse gas emissions for aircraft?

With the new Biden administration likely to rejoin the Paris Climate Accord, it’s worthwhile to consider some of the paths to a sustainable climate future. The transition to a carbon neutral future for ground-based transportation with renewable energy and electrification is underway, but the path for aircraft is much less clear.

Trains are already electrified in much of the world and auto manufacturers are in a race to electrify the majority of vehicles as well. However, electric planes, other than small aircraft, are not feasible as batteries do not provide the same energy density as liquid fuels. Even ocean going ships are facing a challenge to find a fossil fuel replacement.

Flying burns 1.5 billion barrels of jet fuel every year. It is responsible for 12% of all transport CO₂ emissions and 2% of global carbon emissions. Aircraft were largely exempted from the Paris Climate Accord, but are widely cited, especially in Europe, for being a primary contributor.

New research shows that just 1% of the world’s population is responsible for half of all commercial airlines CO₂ emissions.

There are currently four technology strategies being considered to replace fossil fuels for aircraft propulsion in the intervening time frame:

Bio-based direct replacement for kerosene

A carbon closed loop where atmospheric CO₂ is consumed to produce a bio-fuel so zero net emissions occur, presumably with renewable energy powering the logistics. These include:

  • Utilizing second-generation oil-seed crops and waste oils

  • Microalgal oil

  • Using lignocellulosic biomass

The issue with farming oil seed crops to produce a replacement for petroleum fuel is not only the issue of net energy (which could become less of an issue with electric tractors) but potentially displacing food crops. Currently, biodiesel is about 3% of European Union total transport fuels

Neste has airline partners for its sustainable aviation fuel (SAF), produced largely by recycling waste cooking oil. American Airlines (AAL) has committed to purchase 9 million gallons of its product over three years. In 2019, all U.S. airlines consumed about 18 billion gallons. So SAF is a minuscule but important fraction at present.

Exxon Mobil Corp. (XOM) has a long-standing research program on oil producing algae. 

Many efforts have come and gone to scale up algal biofuel, but it is a daunting task to replace current fuels. “Simulations of microalgal biofuel production show that to approach the 10% of (all) EU transport fuels expected to be supplied by biofuels ponds three times the area of Belgium would be needed.” wrote Swansea University Marine Biologist Professor Kevin Flynn in 2017. “And for the algae in these ponds to produce biofuel, it would require fertilizer equivalent to 50% of the current total annual EU crop plant needs.” 

Presumably, genetically engineered micro algae could improve those numbers.

Lignocellulosic biomass can also be converted into biofuel. Globally, there are three major crop residues being used for bioethanol production, that is, rice straw, wheat straw, and corn stover (which in turn can be chemically reacted into jet fuel). 

Forest product waste can also be used. Researchers in Norway have concluded that up to 34% of its forest product waste could be converted into alcohol then reacted into kerosene. The Norwegian government has set a goal that 30% of all jet fuel used in Norway by 2030 will be sustainably produced biofuels. 

Produce a petroleum replacement fuel directly from CO₂

A closed loop approach for petroleum jet fuel replacement is being researched and tested from the catalytic conversion of CO₂ (e.g. from industrial processes or fossil fuel power plants). Synhelion, a European company, is planning to produce jet fuel via a solar powered chemical reactor. Potentially the CO₂ could be captured directly from the atmosphere. Synhelion is partnering with ETH Zurich spinoff Climeworks that captures CO₂ and H₂O directly from air. Another startup company, U.S.-based Carbon Engineering aims to produce an intermediary chemical which in turn be processed into a jet fuel equivalent. But efficiency remains a big question and it needs to all be powered by renewable energy for net emissions improvement.

Instead, replace kerosene with hydrogen

Airbus has a program researching the feasibility of powering future aircraft with hydrogen stored as liquid at cryogenic temperatures. 

Clearly this is not a direct replacement, but one requiring major new aircraft designs Airbus has envisioned, along with new supporting infrastructure. And the hydrogen will need advancement to be produced at scale in a carbon neutral way (e.g. electrolysis of water using solar power). The good news is that when hydrogen is combusted it only produces water vapor.

Ammonia fuel for a substitute sustainable aviation propulsion system

Reaction Engines in the UK has, with funding from Siemens, researched the application to existing aircraft engines modified with heat exchangers and catalysts (to partially react the ammonia to free hydrogen) which together with the ammonia mimics conventional jet fuel. Apparently this does not require major redesign of the aircraft (ammonia can also be stored in the wing tanks) which could speed adoption and it may be possible to retrofit existing jet engines. More research and development is needed.

Hope for technological advancements

While aviation is not currently a major factor in greenhouse gas emissions, if it continues to grow as it did pre-Covid, it will be a concern to overcome when reduction of other sources of CO₂ emissions begin to reach technological diminishing returns in the future. Currently, sustainable replacement fuels for petroleum jet fuels are minuscule compared to the need. Hopefully technology advancements will provide a viable solution for net zero emissions air transportation at near conventional costs. It will take all stakeholders (governments, investors, airlines, aircraft manufacturers, air cargo companies, fuel suppliers, carbon offset suppliers — even for individual travelers) to come together to forge realistic solutions for industry wide net zero emissions flights by 2050.

With less than 30 years to go, there is little time to spare if a major redesign of aircraft and infrastructure is required before existing fleets begin to be replaced. In the meantime, airlines continue to prioritize aircraft and operational efficiency.